Remember, fluorine only has only two core electrons shielding the valenece electrons form the nucleus. The valance electrons are in the second principal quantum energy level, so the valence electrons are close to the nucleus. These two factors combine to strongly attract the electrons to the fluorine nucleus.

On the other hand, iodine has 46 core electrons shielding the valence elctrons from the nuclues. Additionally, the valance electrons are in the fifth principal quantum energy level.